An automatic transmission generally used for an automobile has a torque converter to which engine power is input and a shift gear mechanism driven by the torque converter. In such a transmission, frictional elements are selectively engaged with each other to switch a power transmitting route and thereby establish a desired shift stage. The frictional elements are engaged with or disengaged from each other by an actuator driven by means of hydraulic pressure. Shift stages are switched by controlling hydraulic pressure.
Recently, an attempt to use multiple shift stages has been made to enhance a fuel consumption rate and engine output power. For instance, Japanese Unexamined Patent Public Disclosure (Kokai) No. 61-99745 discloses an automatic transmission in which a main transmission and a sub-transmission, each having independently operated shift gear mechanism are connected in series with each other. These two transmission are driven simultaneously or alternatively to thereby accomplish multiple shift stages.
In addition, U.S. Pat. No. 4,934,215 also discloses an automatic transmission attempting to accomplish multiple shift stages.
These conventional automatic transmissions have, stored therein, predetermined patterns for shift-changing with parameters of a vehicle speed and an engine load. Drive conditions, such as a vehicle speed and an engine load, are checked up with the shift-change patterns to determine whether a shift change should be done. Then according to the determination, a signal ordering a shift change is emitted to establish a desired shift stage. On shift-changing, successive order signals are sometimes emitted to accomplish shift changes over multiple shift stages, for instance, a first signal ordering 4-3 shift down, is emitted and, successively, a second signal, ordering 3-2 shift down, is emitted. It should be noted that hereinbelow a term "M-N shift down" or "M-N shift up" means a shift change from a M shift stage to a N shift stage, and a term "shift down" is used if M is greater than N while a term "shift up" is used if M is smaller than N.
When the successive order signals are emitted as aforementioned, it is usual that a shift change according to a first signal is first accomplished and then a shift change according to a second or later signals is accomplished. However, when such successive order signals are emitted in a short period of time, there arises a problem of response delay, namely time lag between an emission of an order signal and an actual start of a shift change due to response delay of frictional elements.
To solve this problem, a shift gear mechanism can be modified to be controlled so that when a new order signal is output, a shift change according to the new order signal is initially accomplished. However, this modification causes other problems such as, for instance, the problem of giving an uncomfortable feeling to a driver because frictional elements in the process of disengagement return to engagement.
In addition, the aforementioned modification makes a shift change system complicated if the system accomplishes a feed-back control during a shift change for engagement force of frictional elements. For instance, when an order signal indicating 3-2 shift down is emitted before 4-3 shift down has been completed, a shift change is accomplished so that a gear ratio is changed to one for 2nd shift stage while sensing the operation of a shift gear mechanism in real time. Thus, a shift change control cannot avoid being complicated.
In particular, a shift change control is complicated in an automatic transmission having two transmissions connected in series with each other. Each transmission has a shift gear mechanism driven independently of the other. When gear ratios have to be simultaneously changed in both the transmissions, change of a gear ratio in one transmission has to be harmonized with change of a gear ratio in the other transmission in order to prevent shock due to a shift change from occurring. This causes a shift change control to be more complicated than usual. In particular, when a gear ratio is to be increased in one transmission and a gear ratio is to be decreased in the other transmission, a shift change control is extremely complicated.